When you replace the belt, and you should, you have to find a way to hold the flywheel solidly in place while you unscrew the nut in the middle. If you make a thing that looks like a ping pong paddle out of plywood, and put 3 or four bolts through it that catch on the fins on the flywheel, and rotate the flywheel until the "handle" of the paddle jams against a frame part, you're ready to unscrew the nut. Then, finish the job by removing the inner pully and cleaning out the old grease from the rollers in there, and regreasing them. There's a very useful tech tip in www.ch250.net
that will explain this.